Communication systems use high-powered transmitter amplifiers to transmit a communications signal. Any amplifier exhibits nonlinear behavior when operating close to saturation at a maximum output power level. Operating at saturation injects nonlinear distortion. The nonlinearity can be characterized into two types. Amplitude modulation nonlinearity occurs where an incremental change in input signal power does not correspond to a linear incremental change in the output power. Phase modulation nonlinearity occurs when an incremental change in input signal power results in a phase change of the output signal. Operating as close to saturation as possible maximizes the output power and thereby maximizes overall amplifier efficiency. However, these nonlinear effects become more severe closer to saturation and eventually degrade the performance such that a communications link can no longer be maintained.
A decision-directed approach is typically used for m-ary multi-level modulations methods, such as conventional 16-QAM modulation. U.S. Pat. No. 6,154,503 describes an automatic gain control system that responds to baseband signal distortion. The automatic gain control (AGC) system monitors the distortion of the outer constellation points of a complex modulation format such as quadrature amplitude modulation (QAM) and adjusts both the RF and IF AGC gain to minimize such distortion. This receiver contains at least two stages of AGC gain. Typically, the RF AGC amplifier is contained in the tuner that is followed by an IF AGC amplifier. The IF amplifier is coupled to a mixer and an associated low pass filter which, in combination and when driven by a particular frequency, produces a baseband or near baseband signal. The baseband signal is digitized and processed by a gain control circuit. The gain control circuit produces IF and RF AGC signals that control the gain of the respective AGC stages.
Modern cell phone standards rely on high order modulations and require a low BER, and are well suited for decision directed gain control. Automatic gain control is employed on all modern receivers to insure that the received signal level is scaled to the desired amplitude and does not depend on the channel attenuation. The output of an AGC is an AGC adjusted input Y given by Y=A·X=A(I+jQ)), where A is the gain, X is the input having real components I and imaginary components Q. An AGC slicer is used for determining symbol points within a predetermined constellation space from quadrature analog values of an input signal. The AGC may be performed at radio frequency, intermediate frequency, or in the digital domain. Two AGC algorithms are commonly used: power-directed and decision-directed. The error between the desired signal and actual signal for a power-directed method is a function of the average received power P, e=|IIN+jQIN|−P. The error between the desired signal and actual signal for a decision-directed method is e=(IIN−Î)sign(Î)+(QIN−{circumflex over (Q)})sign({circumflex over (Q)}), where Î and {circumflex over (Q)} indicate quadrature decisions. A receiver may employ both algorithms by cascading a power-directed AGC followed by a decision directed AGC.
Signals with variable envelope experience more corruption than do constant-envelope signals. For example, a 16-QAM modulated signal is corrupted more than an 8-PSK signal. A multilevel constellation used in communication systems is impacted by amplifier saturation distortion. Outer points of the constellation are amplified less than the inner points in the constellation and therefore the outer points appear compressed towards the origin of the constellation. Inner points of the constellation are rotated relative to the outer points. This compression and rotation causes bit error rates as the transmitted signals are expected to be in alignment with the constellations but are distorted away from the expected points. The automatic gain control (AGC) subsystems are standard on all modern receivers. The standard AGC subsystem uses gain control algorithms that are suboptimal when a nonlinear power amplifier distorts the received signal. This suboptimal behavior increases the bit error rate (BER). These and other disadvantages are solved or reduced using the invention.